Adult mammalian spermatogenesis is highly dependent upon testosterone. Androgen receptors are localized to Sertoli cells, making it likely that the effects of testosterone are mediated via the Sertoli cell. However, remarkably little is known about how testosterone functions in vivo. Reduced intratesticular testosterone can lead to increased germ cell apoptosis, suggesting that testosterone may serve a cell survival factor. The mechanism(s) by which testosterone protects germ cells from apoptotic death has received very little attention. The Fas system, including Sertoli cell Fas ligand, and Fas, expressed by germ cells, and germ cell Fas, has been implicated in testicular germ cell apoptosis. The Bcl-2 family of proteins plays a central role in the regulation of apoptotic cell death in a variety of cell types. Some among these proteins, including Bcl-2 and Bcl-xlong, inhibit cell death, while others, including Bax and Bcl-xshort, promoter apoptosis. As yet, we do not know which of these proteins is present in particular germ cells, nor how they respond to testosterone. A central hypothesis of this grant is that testosterone promotes the survival of germ cells via its effects on the Sertoli cell/germ cell Fas system, and on the Bcl-2 family of proteins involved in cell survival/cell death. We will first determine the extent to which binding of testosterone by ABP explains the high intratesticular testosterone concentration is required to maintain spermatogenesis in the adult rat. We then will test the concept that testosterone, acting through the Sertoli cell, influences the Fas system and, through this system, cell survival/cell death proteins of the Bcl-2 family, thereby influencing whether or not internucleosomal cleavage and subsequent apoptotic death of germ cells occurs. The results of these studies, taken together with the results of studies proposed by Dr. Jarow in Project I, should elucidate common principles involved in regulating spermatogenesis in rats and men, and at long last should provide concrete information on mechanism(s) by which testosterone acts in vivo to regulate spermatogenesis.